ABSTRACT
The safety margin is determined by the probability that certain types of events will occur and that some events may be more severe than others. For pressurized water reactors, the types of events that can cause a thermal design limit to be exceeded are described in a document called “Nuclear Safety Criteria for the Design of Stationary Pressurized Water Reactor Plants.” The mechanical integrity of the cladding is addressed separately through a different set of requirements that encompasses the mechanical stresses expected to be exerted on the cladding at different burnups corresponding to different fuel pin temperatures and thermal expansion rates. Rod bowing can occur when a fuel rod is heated non-uniformly or when the thermal expansion of the fuel pins is different at different axial locations. The nuclear regulatory commission and other countries in the international nuclear community have agreed to exchange technical information on thermal-hydraulic safety issues related to reactor and plant systems.
